Influence of Carbon Black on the Morphology and Mechanical Properties of Phase‐Separated Synthetic Rubber Blends

Blending of different elastomer types and use of reinforcing particulate fillers are important strategies toward achieving the desired balance of properties in rubber applications. We studied a series of phase‐separated blends of nitrile‐butadiene rubber (NBR) and styrene‐butadiene rubber (SBR) with different ratios between the two rubbers. Compounds filled with carbon black (CB) were investigated and compared to unfilled compounds. Morphology and nanomechanical characteristics were investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and viscoelastic behavior was studied as a function of temperature using a dynamic mechanical analyzer (DMA). The phase‐separated domain size decreased significantly for CB‐filled versus unfilled blends, indicating a compatibilization effect of CB in the rubber blend composites, but without any noted shifting or broadening of the glass transition temperatures. Insights into selective phase partitioning of CB were also provided by AFM and DMA, which explained a 28% reduction in tensile strength relative to additive expectations for the CB‐filled blend of 25% NBR and 75% SBR due to the relative deficiency of CB in the NBR phase.